Similar plastic parts are commonly produced in injection molds with single or multiple cavities. In the case of an injection molding machine wherein the mold has multiple cavities, it is known to use a hotrunner system to deliver the hot plastics material or melt from a melt plastification barrel of the machine to the cavities in the mold. The hotrunner system provides the plastic melt at a defined melt pressure and a controlled melt temperature to each mold cavity. In order to accomplish this objective, the hotrunner system commonly employs a heated manifold through which melt conduits extend and heated injection nozzles.
Nozzle valve gates are used in the aforementioned melt distribution systems to control the opening and closing of gate orifices, that is, the orifices that open into each mold cavity and through which the melt is delivered. The valve gate is a positive shut off device that has an open and closed position. At the beginning of melt injection, a valve pin of the valve gate opens the orifice in order to allow the plastic melt to fill the adjacent cavity. In addition, after the cavity has been filled, the gate orifice remains open during a packing phase which relies on packing pressure to control the quality of the plastic part. While the thermoplastic melt starts to solidify during the packing phase, the valve gate closes the orifice to achieve a clean gate mark on the plastic part surface and to avoid stringing or drooling of melt through the gate from the hotrunner system while the mold opens for part injection.
A melt channel or passage is formed in the nozzle of the valve gate to deliver the hot plastics melt to the gate orifice. Movement of the valve pin inside this melt channel is generally an open and closed stroke in the axial or longitudinal direction of the nozzle. The valve pin is actuated between open and closed positions by means of a valve actuator that is connected to a rear end of the valve pin. With known hotrunner systems, the valve actuator is commonly located externally of the heated components of the hotrunner system (for example, the manifold) because the commonly used valve actuators are not functional at the usual melt processing temperature of thermoplastics materials which is between 200 and 450° C. Generally pneumatic and hydraulic valve actuators are provided with seals between the pistons and their respective cylinders that operate only below 200° C. Also, electromechanic actuators require a low ambient temperature of less than 200° C. It will be understood that a heated melt distribution system or hotrunner system inside a valve gate mold can, depending on the location of the actuators, affect the valve actuators by heat conductivity, radiation and convection. Because of this effect, valve actuators are commonly positioned at a sufficient distance from the heated surface of the melt distribution manifold and the injection nozzle to keep them within their operating temperature range, which is preferably below 100° C. Known valve pin actuators can be physically separated from the heated manifold and the injection nozzle or nozzles by means such as levers, racks, yokes and extended push/pull rods which allow the actuators to be located in a remote location where the actuator temperature can be maintained below 100° C. In addition to this thermal separation from the hotrunner manifold and the nozzles, it is known to provide for direct or indirect cooling of the actuators. Thus a cooling circuit within the injection mold can be directly or indirectly connected with the actuator to withdraw heat from the actuator.
It is also known to provide injection molds with a high number of cavities for making small plastic parts and it is advantageous to make such a mold as compact as possible. However, it is difficult and costly to integrate valve actuators with an effective cooling system in a compact mold of this type. Generally, valve pin actuators require considerable space inside an injection mold and they can add to the overall stack height of the mold. Moreover, forming cutout spaces for the actuators and bores or cutouts for cooling lines as well as air, hydraulic, or electric lines weakens the mold plate structure that has to support the substantial forces of the melt injection pressure inside the mold cavities and the clamping force in the molding machine.
U.S. Pat. No. 6,062,840 which issued May 16, 2000 to Dynisco Hotrunners, Inc., describes a co-injection molding system that employs three position actuators for moving respective valve pins into a closed position, a middle position or an open position in which skin and core material flow is permitted. The actuator includes a first piston slidably mounted in an actuator housing and a second piston attached to the valve pin, slidably mounted within the first piston. This known system has a manifold through which the melt is distributed and a top clamp plate in which the actuators are located. The piston in the actuator is equipped with ring seals extending around its circumference.
In recent U.S. Pat. No. 7,086,852 dated Aug. 8, 2006, each of the valve gate pins is connected at its rear end to a yoke plate located on one side of a central manifold. Opposite end sections of each yoke plate are connected to piston/cylinder actuators by means of actuator rods.
U.S. Pat. No. 5,533,882 describes a hotrunner valve gated system with at least one nozzle housing positioned in a manifold plate. The housing includes a gate orifice and a reciprocal valve stem positioned therein. There is an actuator for moving the valve stem for opening and closing the gate valve and it is designed to maintain the mechanism in a substantially cooled state. This mechanism is positioned coaxially relative to the nozzle housing. The piston of the actuator extends around the circumference of the nozzle and is connected at its mid point to the valve stem by means of a horizontal piston rod. The piston has at least a couple of seal rings between it and the manifold plate or structure that forms the cylinder for the piston.
There is disclosed herein an injection apparatus for injecting hot plastics material into an injection mold which has an actuator for the valve pin wherein the piston of the actuator is connected to the rear end of the valve pin and wherein the actuator can be used in relatively high temperature plastics injection temperature conditions because no fluid seals are required between the piston and the wall of the actuator chamber in which the piston moves.